/*
 * Copyright 2013 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.rockyang.blockj.base.crypto;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.io.*;
import java.security.Provider;
import java.security.SecureRandomSpi;
import java.security.Security;

/**
 * Implementation from
 * <a href="https://github.com/bitcoinj/bitcoinj/blob/master/core/src/main/java/org/bitcoinj/crypto/LinuxSecureRandom.java">BitcoinJ implementation</a>
 *
 * <p>A SecureRandom implementation that is able to override the standard JVM provided
 * implementation, and which simply serves random numbers by reading /dev/urandom. That is, it
 * delegates to the kernel on UNIX systems and is unusable on other platforms. Attempts to manually
 * set the seed are ignored. There is no difference between seed bytes and non-seed bytes, they are
 * all from the same source.
 */
public class LinuxSecureRandom extends SecureRandomSpi {
	private static final FileInputStream urandom;

	private static class LinuxSecureRandomProvider extends Provider {
		public LinuxSecureRandomProvider()
		{
			super("LinuxSecureRandom", 1.0, "A Linux specific random number provider that uses /dev/urandom");
			put("SecureRandom.LinuxSecureRandom", LinuxSecureRandom.class.getName());
		}
	}

	private static final Logger log = LoggerFactory.getLogger(LinuxSecureRandom.class);

	static {
		try {
			File file = new File("/dev/urandom");
			// This stream is deliberately leaked.
			urandom = new FileInputStream(file);
			if (urandom.read() == -1) {
				throw new RuntimeException("/dev/urandom not readable?");
			}
			// Now override the default SecureRandom implementation with this one.
			int position = Security.insertProviderAt(new LinuxSecureRandomProvider(), 1);

			if (position != -1) {
				log.info("Secure randomness will be read from {} only.", file);
			} else {
				log.info("Randomness is already secure.");
			}
		} catch (FileNotFoundException e) {
			// Should never happen.
			log.error("/dev/urandom does not appear to exist or is not openable");
			throw new RuntimeException(e);
		} catch (IOException e) {
			log.error("/dev/urandom does not appear to be readable");
			throw new RuntimeException(e);
		}
	}

	private final DataInputStream dis;

	public LinuxSecureRandom()
	{
		// DataInputStream is not thread safe, so each random object has its own.
		dis = new DataInputStream(urandom);
	}

	@Override
	protected void engineSetSeed(byte[] bytes)
	{
		// Ignore.
	}

	@Override
	protected void engineNextBytes(byte[] bytes)
	{
		try {
			dis.readFully(bytes); // This will block until all the bytes can be read.
		} catch (IOException e) {
			throw new RuntimeException(e); // Fatal error. Do not attempt to recover from this.
		}
	}

	@Override
	protected byte[] engineGenerateSeed(int i)
	{
		byte[] bits = new byte[i];
		engineNextBytes(bits);
		return bits;
	}
}